The present invention relates to a method and a toner for producing images for use as in electronic photography, electrostatic recording and electrostatic printing.
In toner-based image production technology as electronic photography, electrostatic recording and electrostatic printing, images are formed of toner on a support like paper, followed by fixing to produce such fixed images as photocopy and photoprint. A number of fixing methods have been known and practiced. Among them are:
The hot roller fixing method in which the toner is softened with heat and pressure under a hot roller and then is allowed to cool to solidify;
The flash fixing method in which the toner is heated and melted by a flash light as from a xenon lamp, followed by cooling to solidify; and
The pressure fixing method in which the toner is plastic deformed and fixed under pressure by pressure roller without heat.
The hot roller fixing method is now dominantly practiced on the strength of its advantage over the flash fixing method with regard to thermal conductivity or energy efficiency. The toner used in the hot roller fixing method is one prepared by melt-kneading such a toner binder as styrene resin and polyester together with a colorant, followed by pulverizing. The problem with this method is that the toner will melt and stick to the hot roller (hot offset) if the temperature is too high. If the temperature of the hot roller is too low, on the other hand, the toner will not soften enough and fix well. The tendency is toward to energy saving and size reduction in equipment like copying machines. Reflecting that tendency, there is a mounting call for a higher hot offsetting temperature and a lower temperature fixability.
Multi-color copying machines and multi-color printers in particular require a toner with a lower melt viscosity because a high degree of gloss and color miscibility are needed in images. A toner binder of the sharp-melting polyester type is now used.
Some ideas which it is claimed satisfy those two requirements in hot roller fixingxe2x80x94a low temperature fixability and a high hot offset resistancexe2x80x94are proposed, including (a) one using a polyester-type toner binder partially crosslinked with a multifunctional monomer (unexamined Japanese patent publication No. xe2x80x9cShoxe2x80x9d57-109825), (b) one using a toner binder with a molecular weight distribution having two peaks, one of low molecular weight and the other of high molecular weight (examined Japanese patent publication No. xe2x80x9cShoxe2x80x9d63-32180), and (c) one in which a releasing component as wax and polysiloxane is added to the toner (unexamined Japanese patent publication No. xe2x80x9cShoxe2x80x9d60-8309 and unexamined Japanese patent publication No. xe2x80x9cHeixe2x80x9d5-197202). For multi-color equipment, furthermore, a releasing agent as silicone oil has been applied on the hot roller, because the toner in itself is liable to cause hot offset trouble.
However, the toners disclosed in unexamined Japanese patent publication No. xe2x80x9cShoxe2x80x9d57-109825 and examined Japanese patent publication No. xe2x80x9cShoxe2x80x9d63-32180 are indeed improved in hot offset resistance, but the low temperature fixability is sacrificed, and can never be used in multi- color image production because no luster will develop. The other toners disclosed in unexamined Japanese patent publications Nos. xe2x80x9cShoxe2x80x9d60-8309 and xe2x80x9cHeixe2x80x9d5-197202 are improved in hot offset resistance in black and white, but are not so high in hot offset resistance that oil-less fixing is possible in multi-color image production. The other idea that silicone oil is applied to the hot roller requires additionally an oil tank and an oil applicator, making the equipment complicated and large. What is more, the oil causes the hot roller to degrade, requiring periodical maintenance. Furthermore, oil inevitably sticks to copying paper, overhead projector (OHP) film and other parts. Stuck oil can result in a problem of deteriorated color tone on the OHP film in particular.
The flash fixing method is free from hot offset trouble unlike the hot roller fixing method because the toner does not come in direct contact with the heat source. It has also an advantage over the hot roller fixing method in that the fixing rate is high. The toner used for the flash fixing is one prepared by melt-kneading and pulverizing a toner binder as low softening point polyester and epoxy resins with a colorant.
The problem with the conventional flash fixing method is its poor light-to-heat conversion efficiency, however. Because of that, a high energy lamp is required even on a low softening point toner, which is bound to a large size of equipment and a very large electricity consumption. In addition, color toners other than the black one are low in absorption of flash light energy and are difficult to sufficiently melt and fix. Because of different light absorption rates with colors, the toner can hardly be used in multi-color image production. Attempts have been made to raise the light-to-heat conversion efficiency in the flash fixing method as exemplified by an idea of adding infrared absorber (d) (unexamined Japanese patent publication No. xe2x80x9cShoxe2x80x9d58-102248). But this idea proves to be not a satisfactory solution and can not be applied in multi-color image production.
The pressure fixing method has an advantage over the thermal roller fixing method and the flash fixing method in that no heat source is required. That is, the pressure fixing method can save energy and needs no warm up time. The toner used in the pressure fixing method has to be fixed by plastic deformation alone. For the purpose, a low softening point substance easy to deform at room temperature is proposed which is capsulated with a polymer with a high hardness and a high softening point.
With such a toner possible to deform without application of heat, however, the fixed image is also liable to deform at room temperature or poor in durability, heat resistance and wear resistance. No toner of that kind has been put to practical use yet. Besides, the fixed image would be insufficient in gloss and could hardly be used in multi-color image production.
In any of those fixing methods, furthermore, a toner binder with a low viscosity or a low number average molecular weight is used to facilitate the fixing process. But that brings about some problems with the fixed image, that is, insufficient fixing strength, bending resistance (quality of toner not peeling off when the support is bent) and storage stability (quality of toner staying fixed on the support for a long time). The problem is serious especially in multi-color image production where a toner binder with a still lower viscosity is used to enhance gloss.
In addition to the foregoing three fixing methods, an ultraviolet or visible ray fixing method (e) is proposed in which a photo-curing resin is used (unexamined Japanese patent publication No. xe2x80x9cShoxe2x80x9d57-144563). With no heat applied, however, the fusion bond between toner particles is insufficient in this method, and no satisfactory results can be achieved with the fixed image with regard to fixing strength, bending resistance, storage stability and gloss.
It is an object of the present invention to provide a method of producing images by low energy fixing process without such problems as hot offset and a toner suitable for the purpose, and more particularly a method of producing images which does not require a hot roller to fix or, if so, needs no application of silicone oil, yet produces a high degree of gloss in multi-color image production and a toner suited to this fixing method.
It is another object of the present invention to provide a method of forming fixed images excellent in fixing strength, bending resistance and storage stability and a toner adapted to that.
To achieve the foregoing objects, the present invention provides a toner comprising a toner binder and a colorant, the toner binder composed of a resin (I) selected from among cation-polymerizable thermoplastic resin (A) and radical-polymerizable thermoplastic resin (B) and the preparation having a glass transition point (Tg) before curing of not lower than 35xc2x0 C.
The method of producing images according to the present invention is characterized in that an unfixed image of a toner made of a toner binder and a colorant, which is formed sticking on a recording material, is fixed by a combination of softening the toner by heat and the subsequent curing.
In the present invention, the resins making up the toner binder are polymerizable ones and are cured in fixing the image on the support so that the number average molecular weight can be increased after the fixing step. That is, a toner binder with a low number average molecular weight can be used, and fixed at a low temperature and with low energy. Because of the low number average molecular weight, also, the toner is low in viscosity and excellent in gloss development and can produce high-quality multi-color images.
Since the toner according to the present invention polymerizes and cures to increase in number average molecular weight while fixing, the hot offset is hardly caused, with a high fixing strength and an excellent wear resistance developed.